Method of heat treatment of silicon wafer doped with boron

ABSTRACT

A method of subjecting a silicon wafer doped with boron to a heat treatment in an argon atmosphere, wherein the argon atmosphere is replaced with a hydrogen atmosphere or a mixed gas of an argon gas and a hydrogen gas in a proper fashion, to thereby uniformize a boron concentration in the thickness direction of the surface layer of the silicon wafer doped with boron.

TECHNICAL FIELD

[0001] The present invention relates to a method for a heat treatment ofa silicon wafer doped with boron in an argon atmosphere, which iseffective in uniformizing the boron concentration in the thicknessdirection of the wafer surface layer.

BACKGROUND ART

[0002] It is generally observed that a silicon wafer cut out from adislocation-free silicon monocrystal ingot grown by the Czochralski (CZ)method has a crystal defect called COP (Crystal Originated Particle).The COP is caused by voids introduced during the ingot growing process.When voids are exposed on the surface of the silicon wafer to form pitsor, even if not exposed on the surface, when they are contained in thesurface layer of the silicon wafer (i.e., when the COP is contained in adevice activation area), they become a factor of causing a defect in thedevice properties, such as a defect in withstand voltage in the gateoxide film or a defect in leak characteristic.

[0003] In view of the circumstances described above, in order to removethe COP which is exposed on the surface of the silicon wafer or includedin the surface layer, a heat treatment (annealing) is conducted in anonoxidizing atmosphere such as a hydrogen atmosphere or an argonatmosphere at a high temperature of approximately 1200° C. (e.g.,Japanese Patent Application Laid-Open Publications No. 58-85534 and No.4-167433).

[0004] The above heat treatment (annealing) actually reduces oreliminates the COP exposed on the surface of the silicon wafer orcontained in the surface layer by diff-using oxygen from the wafersurface layer to the outside.

[0005] But, when the silicon wafer having boron as a dopant is annealedin a hydrogen atmosphere, not only oxygen but also boron is diffusedfrom the wafer surface layer to the outside. Thus, there is a drawbackthat a resistivity in the vicinity of the wafer surface layer becomesdifferent from the initially assumed one.

[0006] In this connection, Japanese Patent Application Laid-OpenPublication No. 10-144698 discloses a method of treating a silicon wafercontaining boron in an argon atmosphere with almost no change in adistribution of boron based on a discovery made by the inventors thatwhen the silicon wafer containing boron is annealed in the argonatmosphere, boron is hardly diffused to the outside but only oxygen isselectively diffused to the outside.

[0007] However, the invention described in Japanese Patent ApplicationLaid-Open Publication No. 10-144698 is strictly conducted as apretreatment of epitaxial growth. In practice, it cannot fullyuniformize the boron concentration in the thickness direction of thewafer surface layer.

DISCLOSURE OF THE INVENTION

[0008] The prevent invention has been made in view of the abovecircumstances and provides a method which can fully uniformize a boronconcentration in a thickness direction of a surface layer of a siliconwafer doped with boron.

[0009] In order to achieve the above object, the present inventors havemade a devoted study and found that when the silicon wafer doped withboron is annealed in argon, the boron concentration in the thicknessdirection of the surface layer of the silicon wafer increasestemporarily and then lowers gradually.

[0010] It is said that when annealing in argon is conducted, the boronconcentration in the thickness direction of the vicinity of the surfacelayer becomes uniform. And, there are commercially available siliconwafers having a uniform boron concentration. But, such silicon waferswere also observed that the boron concentration in the wafer surfacelayer increased temporarily and lowered gradually.

[0011] As to the phenomenon of temporal increase in the boronconcentration, the present inventors have found that the boronconcentration in the thickness direction of the surface layer of thesilicon wafer can be uniformized by appropriately applying an effect oflowering the boron concentration by annealing in hydrogen to removeexcessive boron contained in the wafer surface layer and then annealingin argon, and achieved the present invention.

[0012] The present inventors consider that the temporal increase inboron concentration in the surface layer of the silicon wafer is causedby adhesion of boron to the silicon wafer because of some cause prior tocharging into a heat treatment furnace.

[0013] More specifically, the present invention provides the following.

[0014] (1) A method of subjecting a silicon wafer doped with boron to aheat treatment in an argon atmosphere, wherein the argon atmosphere isreplaced by a hydrogen atmosphere or “a mixed gas of an argon gas and ahydrogen gas” in a proper fashion, to thereby uniformize the boronconcentration in the thickness direction of the surface layer of “thesilicon wafer doped with boron”.

[0015] The “wafer surface layer” is basically assumed to have a range ofwithin 5 μm from the wafer surface, and more preferably within 10 μm.

[0016] (2) A method of subjecting a silicon wafer doped with boron to aheat treatment in an argon atmosphere, wherein the heat treatment isperformed in the initial stage of the heat treatment with the argonatmosphere replaced by a hydrogen atmosphere or “a mixed gas of an argongas and a hydrogen gas” in a proper fashion, to thereby removecontamination of the boron from a surface of the silicon wafer dopedwith boron in a stage prior to the heat treatment and to perform asubsequent heat treatment in the argon atmosphere.

[0017] The heat treatment in the hydrogen atmosphere or “the mixed gasof an argon gas and a hydrogen gas” performed in a proper fashion in theinitial stage of the heat treatment in a nonoxidizing atmosphere is touniformize the boron concentration in the thickness direction of thewafer surface layer.

[0018] The “initial stage of the heat treatment” is a concept includingboth of from the initial time of the heat treatment of the silicon wafer(when hydrogen is flown in the heat treatment furnace from the beginningof the heat treatment and replaced by argon in due course to perform theheat treatment) and the initial certain period of the heat treatment ofa silicon wafer (when argon is first flown into the heat treatmentfurnace and replaced by hydrogen in due course to perform the heattreatment, then replaced by argon again to continue the heat treatment).Excessive boron contained in the wafer surface layer is removed byperforming the heat treatment in hydrogen in the above stage touniformize the boron concentration in the thickness direction of thewafer+surface layer, and the annealing in argon is performed. And, theboron concentration in the thickness direction of the wafer surfacelayer is kept in the uniform state by annealing in argon. As a result, asilicon wafer with the uniformized boron concentration in the thicknessdirection of the surface layer can be obtained by the present invention.

[0019] (3) The method according to (1) or (2) above, wherein “the mixedgas of an argon gas and a hydrogen gas” is “a mixed gas of an argon gasand a hydrogen gas” containing the hydrogen gas in a concentration of anexplosion limit or below.

[0020] (4) A method of annealing in argon with a hydrogen gas addedto-uniformize a boron concentration in a thickness direction of asurface layer of a silicon wafer doped with boron.

[0021] In the above (4), “with a hydrogen gas added to uniformize aboron concentration in the thickness direction of the surface layer of asilicon wafer doped with boron” means, for example, “addition ofhydrogen in the initial stage of annealing in argon to perform the heattreatment with hydrogen”.

[0022] In that case, the hydrogen heat treatment in the initial stagecan be performed at a temperature in a range of, for example, 800° C. to1200° C. For example, when the hydrogen heat treatment is performed atless than 800° C. while the temperature of the heat treatment furnace isbeing increased (e.g., hydrogen is added at 300° C., annealing inhydrogen is performed at 300° C. to 500° C., and switching to the argonatmosphere is made at 500° C.), it is not practical because an effect ofremoving boron from the wafer surface layer by annealing in hydrogendelays. Meanwhile, for example when the heat treatment in hydrogen isperformed at a temperature exceeding 1200° C. while the temperature ofthe heat treatment furnace is being increased (e.g., hydrogen is addedat 1210° C., annealing in hydrogen is performed at 1210° C. to 1300° C.,and switching to the argon atmosphere is made at 1300° C.), annealing inargon is made before reaching 1200° C., and the effect by annealing inargon is produced in advance. Thus, the effect of removing boron byannealing in hydrogen intended by the present invention may not beprovided satisfactorily.

[0023] The above conditions are variable depending on the heat treatmentconducted in 100% of hydrogen, the heat treatment conducted in the mixedgas with argon or time of annealing in hydrogen, but the presentinvention can be performed at a temperature of 800° C. to 1200° C. evenin 4% of hydrogen which is an explosion limit of hydrogen. And, when theheat treatment is performed in hydrogen in concentration of 100%, theinvention can be performed at a temperature less than 800° C., and thehydrogen heat treatment time becomes short.

[0024] In either case, the condition of “the heat treatment in hydrogenin the initial stage” according to the present invention is determinedby totally considering a wafer to be obtained, a state of the apparatusand others.

[0025] (5) A method of subjecting a silicon wafer doped with boron to aheat treatment in a nonoxidizing atmosphere while uniformizing a boronconcentration in a thickness direction of a surface layer of “thesilicon wafer doped with boron” using the boron adhered to the surfaceof the silicon wafer.

[0026] The “nonoxidizing atmosphere” means an ordinary nonoxidizingatmosphere, such as a hydrogen atmosphere or an argon atmosphere, usedto eliminate the COP in the vicinity of the surface layer of the siliconwafer.

[0027] (6) A method of using a hydrogen gas or “a mixed gas of an argongas and a hydrogen gas” to uniformize a boron concentration in athickness direction of a surface layer of “a silicon wafer doped withboron”.

[0028] (7) The method according to any one of (1) to (6) above, wherein“the silicon wafer doped with boron” is a silicon wafer of silicon dopedwith boron in a non-high concentration.

[0029] The “non-high concentration” used in the present invention meansa boron concentration of preferably 1×10¹⁶ atoms/cm³ or below, and morepreferably 1×10¹⁵ atoms/cm³ or below, and most preferably 1×10¹⁴atoms/cm³ or below.

[0030] (8) A silicon wafer produced by the method according to any oneof (1) to (5) above, wherein “the silicon wafer doped with boron” is asilicon wafer of silicon doped with boron in a non-high concentration.

[0031] (9) A method of subjecting a silicon wafer doped with boron to aheat treatment in an argon atmosphere, wherein when a surface of thesilicon wafer is contaminated with boron in a stage prior to the heattreatment, the heat treatment is performed in hydrogen or “a mixed gasof an argon gas and a hydrogen gas” in the initial stage of the heattreatment in the argon atmosphere, to thereby uniformize a boronconcentration in a thickness direction of a surface layer of “thesilicon wafer doped with boron” after the heat treatment.

[0032] Thus, when the heat treatment is performed in hydrogen or the“mixed gas of an argon gas and a hydrogen gas” in the initial stage ofthe heat treatment in the argon atmosphere, contamination of boron isremoved from the surface of the silicon wafer, and annealing isperformed with the wafer surface cleaned. And, in this case, after thewafer surface is cleaned, annealing is performed in the argonatmosphere, that boron is less diffused to the outside, to uniformizethe boron concentration in the thickness direction of the wafer surfacelayer.

BRIEF DESCRIPTION OF THE DRAWINGS

[0033]FIG. 1 is a graph showing a change in boron concentration in awafer thickness direction at the time when silicon wafers doped withboron and having different initial resistivities were annealed in argon;

[0034]FIG. 2 is a graph showing a change in boron concentration in awafer thickness direction at the time when silicon wafers doped withboron and having different initial resistivities were annealed inhydrogen;

[0035]FIG. 3 is a graph showing results obtained when a silicon waferdoped with boron at a non-high concentration was annealed in argon witha heat treatment temperature changed to 1100° C., 1150° C., and 1200° C.for a prescribed heat treatment period (one hour);

[0036]FIG. 4 is a graph showing results obtained when a silicon waferwas annealed in argon at a prescribed heat treatment temperature (1150°C.) with a heat treatment period changed to one hour, four hours andeight hours;

[0037]FIG. 5 is a graph showing the boron concentration distributionobtained when a silicon wafer was annealed in argon with hydrogen addedin the initial stage of the heat treatment, the boron concentrationdistribution obtained by treating with argon only, and the boronconcentration distribution obtained by treating with hydrogen only.

[0038]FIG. 6 is a graph showing the boron concentration distributionobtained when the added hydrogen concentration at the initial state was100% and. a heat treatment temperature at the time of adding hydrogenwas 800° C.;

[0039]FIG. 7 is a graph showing the boron concentration distributionobtained when the added hydrogen concentration in the initial stage was4% and a heat treatment temperature at the time of adding hydrogen was800° C.;

[0040]FIG. 8 is a graph showing the boron concentration distributionobtained when the added hydrogen concentration in the initial stage was100% and a heat treatment temperature at the time of adding hydrogen was1000° C.;

[0041]FIG. 9 is a graph showing the boron concentration distributionobtained when the added hydrogen concentration in the initial stage was4% and a heat treatment temperature at the time of adding hydrogen was1000° C.;

[0042]FIG. 10 is a graph showing the boron concentration distributionobtained when the added hydrogen concentration in the initial stage was100% and a heat treatment temperature at the time of adding hydrogen was1200° C.; and

[0043]FIG. 11 is a graph showing the boron concentration distributionobtained when the added hydrogen concentration in the initial stage was4% and a heat treatment temperature at the time of adding hydrogen was1200° C.

BEST MODE FOR CARRYING OUT THE INVENTION

[0044] Embodiments of the invention will be described. FIG. 1 is a graphshowing a change in boron concentration in the wafer thickness directionat the time when “silicon wafers doped with boron” and having differentinitial resistivities were annealed in argon, and FIG. 2 is a graphshowing a change in boron concentration in the wafer thickness directionat the time when “silicon wafers doped with boron” and having differentinitial resistivities were annealed in hydrogen.

[0045] The boron concentration was detected by an SIMS (Secondary IonMass Spectroscopy).

[0046] In this embodiment, the silicon wafer used is a P-typeboron-doped silicon wafer formed on a (100) plane with a diameter of 200mm. The P-type boron-doped silicon wafer has an oxygen concentration of10.0 to 15.0×10¹⁷ atoms/cm³ and a resistivity of 1 to 30 Ω-cm.

[0047] As indicated by sample A shown in FIG. 1, when a silicon waferdoped with boron of a non-high concentration (a silicon wafer which isdoped with boron to a non-high concentration) is annealed in argon,there is observed a phenomenon that the boron concentration in the waferthickness direction is temporarily increased and then gradually loweredat a given portion in the wafer surface layer. The occurrence of such aphenomenon regardless of the annealing temperature or the annealing timewas confirmed by the present inventors.

[0048] Meanwhile, it is generally known well that boron is diffused tothe outside after the annealing in hydrogen, and its concentration inthe surface layer is decreased as shown in FIG. 2.

[0049] But, the distribution of boron concentration in the surface layermay be uniform when annealed in argon but the boron concentration oftenincreases in the surface layer as shown in FIG. 1. In this connection,it is described in Japanese Patent Application Laid-Open Publication No.10-144698 that the diffusion of boron to the outside by the heattreatment in an argon atmosphere is hard, but the reason for thephenomenon that the boron concentration increases is unknown.

[0050] Under the circumstances described above, the present inventorshave conducted annealing in argon with the treatment temperature andtime varied in order to obtain clues to the cause of the increase inboron concentration in the surface layer. As a result, it was found thatthe boron concentration in the surface layer increases at all levels,but the concentration near the surface lowers as the temperature becomeshigh and the time becomes long, and the concentration is diffused tobecome high toward the inside (FIG. 3 and FIG. 4).

[0051] It suggests that boron already adhered in the stage before theannealing in argon has diff-used to the inside because boron is notalways supplied from the outside of the wafer during the heat treatment.On the other hand, it is presumed that the boron concentration in thesurface layer lowers when annealed in hydrogen because boron adhered tothe surface reacts with hydrogen and removed during the annealing, andthe boron in the surface layer is diffused to the outside of the wafer.

[0052] Therefore, it was considered that uniform boron distribution canbe obtained by adding hydrogen or gas containing hydrogen to separatethe boron adhered to the surface layer at a moment in a preliminary stepof holding at 1200° C. in annealing in argon and then treating withargon. And, a desired result was obtained by performing the followingexperiment.

[0053] Embodiment 1:

[0054] Switching to hydrogen or gas containing hydrogen (argon dilution)was performed with timing as shown in Table 1, each gas was kept at eachtemperature for 24 minutes, and annealing in argon was continued at1200° C.

[0055] The used wafer was a P-type (boron doped) with a diameter of 200mm and having a resistivity of 10 to 20 (Ω·cm). TABLE 1 CZ wafer, 200 mmP-type, Resistivity: 10-20 (Ω · cm) Level Hydrogen addition temperature(° C.) Added hydrogen ratio (%) 1 800 100 2 800 4 3 1000 100 4 1000 4 51200 100 6 1200 4

[0056] One example of the SIMS analysis results obtained by annealing isshown in FIG. 5. Fore comparison, FIG. 5 shows the boron concentrationdistribution after treating with argon only and the boron concentrationdistribution after treating with hydrogen only. It is seen from FIG. 5that a boron concentration in the surface layer is uniformized byannealing in argon with hydrogen added in the initial stage of heattreatment.

[0057] Then, the boron concentration distribution was examined whenhydrogen was added in concentrations of 4% and 100% in the initial stagewith a heat treatment temperature varied to 800° C., 1000° C. and 1200°C. The results are shown in FIG. 6 to FIG. 11.

[0058] The results shown in FIG. 6 to FIG. 11 are summarized as follows.

[0059] 1) The boron concentration in the surface layer could besuppressed from increasing by adding the hydrogen gas in the preliminarystep of holding at a high temperature to perform annealing in argon.

[0060] 2) For timing to switch to the hydrogen gas, a temperature of800° C. or higher is sufficient when 100% of hydrogen is added.

[0061] 3) When 4% of hydrogen is added, boron in the surface has aslight increasing trend but it is effectively suppressed.

[0062] As described above, when the silicon wafer doped with boron in anon-high concentration is annealed in argon, the boron concentrationincreases temporarily at a given portion in the wafer surface layer, sothat the argon atmosphere is appropriately switched to the hydrogenatmosphere or “the mixed gas of an argon gas and a hydrogen gas” inagreement with the timing of the formation of such a portion. Thus, theboron concentration in the thickness direction of the surface layer ofthe silicon wafer doped with boron can be uniformized.

[0063] Otherwise, when the silicon wafer doped with boron in a lowconcentration is subject to a heat treatment in an atmosphere of “amixed gas of an argon gas and a hydrogen gas”, it is considered that theboron concentration in the wafer thickness direction of the surfacelayer of the silicon wafer doped with boron can be uniformized byvarying a mixing ratio of the hydrogen gas in the “mixed gas of an argongas and a hydrogen gas”.

[0064] In this connection, to perform the present invention, a heattreatment furnace for annealing in hydrogen is basically used becausethe invention includes a heat treatment process with a hydrogen gas, butan explosion-proof construction required when the hydrogen gas is usedbecomes unnecessary by controlling the “mixed gas of an argon gas and ahydrogen gas” to contain the hydrogen gas in a concentration(concentration of 4% or below) of an explosion limit or below, and theheat treatment furnace as the whole is made to have a smaller heatcapacity. Thus, the heat treatment furnace having structurespecifications capable of quickly raising or lowering a temperature canbe provided. And, it becomes possible to reduce the annealing time.

[0065] As described above, the present invention can fully uniformizethe boron concentration in the thickness direction of the wafer surfacelayer of the silicon wafer doped with boron in a non-high concentration.

1. A method of subjecting a silicon wafer doped with boron to a heattreatment in an argon atmosphere, wherein the argon atmosphere isreplaced by a hydrogen atmosphere or a mixed gas of an argon gas and ahydrogen gas in a proper fashion, to thereby uniformize a boronconcentration in a thickness direction of a surface layer of the siliconwafer doped with boron.
 2. A method of subjecting a silicon wafer dopedwith boron to a heat treatment in an argon atmosphere, wherein the heattreatment is performed in the initial stage of the heat treatment withthe argon atmosphere replaced by a hydrogen atmosphere or a mixed gas ofan argon gas and a hydrogen gas in a proper fashion, to thereby removethe contamination of boron from a surface of the silicon wafercontaminated with boron in the stage prior to the heat treatment and toperform a subsequent heat treatment in the argon atmosphere.
 3. Themethod according to claim 1 or 2, wherein the mixed gas of the argon gasand the hydrogen gas is a mixed gas of an argon gas and a hydrogen gascontaining the hydrogen gas in a concentration of an explosion limit orbelow.
 4. A method of annealing in argon with a hydrogen gas added touniformize a boron concentration in a thickness direction of a surfacelayer of a silicon wafer doped with boron.
 5. A method of subjecting asilicon wafer doped with boron to a heat treatment in a nonoxidizingatmosphere while uniformizing a boron concentration in a thicknessdirection of a surface layer of the silicon wafer doped with boron usingthe boron adhered to the surface of the silicon wafer.
 6. A method ofusing a hydrogen gas or a mixed gas of an argon gas and a hydrogen gasto uniformize a boron concentration in a thickness direction of asurface layer of a silicon wafer doped with boron.
 7. The methodaccording to any one of claims 1 to 6, wherein the silicon wafer dopedwith boron is a silicon wafer of silicon doped with boron in a non-highconcentration.
 8. A silicon wafer produced by the method according toany one of claims 1 to 5, wherein the silicon wafer doped with boron isa silicon wafer of silicon doped with boron in a non-high concentration.9. A method of subjecting a silicon wafer doped with boron to a heattreatment in an argon atmosphere, wherein when a surface of the siliconwafer is contaminated with boron in a stage prior to the heat treatment,the heat treatment is performed in hydrogen or a mixed gas of an argongas and a hydrogen gas in the initial stage of the heat treatment in theargon atmosphere, to thereby uniformize a boron concentration in athickness direction of a surface layer of the silicon wafer doped withboron after the heat treatment.